Purification of thiophenols by treatment with aluminum and magnesium alkoxides



United States i latent PURIFICATION OF THIOPHENOLS BY TREAT- MENT WITHALUMINUM AND MAGNESIUM ALKOXIDES Robert J. Laufer, Pittsburgh, Pa.,assignor to Consolidation Coal Company, Pittsburgh, Pa., a corporationof Pennsylvania This invention relates to the art of separatingthiophenols from tar acids. More particularly it relates to purifyingthiophenols containing tar acids by selective reaction of the tar acidswith alkoxides'of aluminum and magnesium.

By tar acids I refer to those constituents present in coal-tardistillates, certain cracked petroleum distillates. and the like, oftenreferred to collectively as phenols, which are soluble in dilute causticsoda, giving sodium cresylate. I particularly refer to a mixture of thesimpler monohydric phenols boiling below about 230 C. and consistingalmost entirely of a mixture of phenol, methylphenols (cresols) anddimethylphenols '(xylenols), with lesser amounts occasionally present ofethylphenols and trimethylphenols.

At present, thiophenols are principally obtained from two sources: as aby-product from the caustic extraction of petroleum distillates and bysynthetic methods starting with benzene. In one widely used directsynthesis technique, the benzene is converted to benzene sulfonylchloride by treatment with a molar excess of chlorosulfonic acid. Theproduct is then converted to thiophenol by reduction in the presence ofa metal-acid system. While the resulting product is of high purity andparticularly usefulfor paint, dyestuffs and pharmaceutical applications,it is relatively expensive. This prevents its wide-spread use for manyapplications.

In obtaining thiophenols from petroleum distillates resulting fromoil-cracking processes, the tar acids and thiophenols are recovered byextracting the petroleum distillate with aqueous caustic solution toproduce watersoluble tar acid salts. In this process of extraction,thiophenols present are also removed by the aqueous caustic solutioninasmuch as the thiophenols are even stronger acids than the phenols ortar acids themselves. The quantity of thiophenols in'the original sourcematerial varies widely, being sometimes as little as one percent byweight of the phenols and ranging as high as 25 percent and above. Thethiophenols consist principally of thiophenol itself and mixedthiocresols and thioxylenols.

In US. Patent 2,767,220, a process is set forth for purifyingthiophenol-contaminated tar acids that are substantially free of neutralhydrocarbon oils. In this process, the feedstock, consisting principallyof tar acids and thiophenols, is contacted with aqueous methanol andwith a low boiling paraflinic naphtha fraction in a continuouscountercurrent extraction zone. The aqueous methanol solution dissolvessubstantially all the tar acids, and the naphtha fraction dissolves thethiophenols. The naphtha fraction is distilled off to yield thethiophenols as still bottoms. Inasmuch as the purification of the taracids is the desideratum of this process, the thiophenols recovered fromthe parafiinic naphtha fraction invariably contain from 2 to 20 percenttar acids by weight. Complete removal of the tar acids from thethiophenols does not take place even when the thiophenols arefractionally distilled in a highly eflicient column, such as a SO-platepacked tower. Thiophenol from such a distillation contains approximately1.5 percent phenol. .Eflicient fractional distillation is likewiseunsuccessful in effecting removal of the tar acids from the mixedthiocresols and thioxylenols. These thiophenols contain even higheramounts of close=boiling tar acids.

Accordingly, it is an object of the present invention'to provide athiophenol of improved purity with respect to tar acid content. It is afurther object to provide a method for obtaining thiophenols free fromtar acids that is readily adaptable to existing techniques forextracting tar acids from petroleum distillate fractions.

In accordance with this invention, a mixture of thin phenols and taracids is treated with an alkoxide selected from the group consisting ofaluminum alkoxides and magnesium alkoxides so that tar acids presentselectively react with the alkoxide to form the'corresponding phenoxide.The thiophenols are then separated from the relatively nonvolatilephenoxides, preferably by distillation. Preferred alkoxides, oralcoholates, are'those formed by reacting aluminum or magnesiummetalwith a lower monohydroxy paraflinic alcohol having from 1 to 4carbon atoms in its chain. Preferred alcohols include methyl, ethyl,n-propyl, isopropyl, n-butyl, isobutyl, secbutyl, and tert-butylalcohols. Magnesium methoxide a particularly preferred reactant.

It has been found that this method is suitable for rendering thiophenolscontaining t-ar acids in amounts up to 50 percent by weight of themixture substantially free from tar acid contamination. The method isparticularly preferred where the thiophenol has a tar acid content of upto 20 percent by weight of the mixture. =In general, where thiophenolitself is to be purified, the contaminant present is usually phenol. Itis believed that an azeotrope is formed thereby preventing a separationof the thiophenol and phenol by conventional distillation techniques.Mixed thiocresols are usually contaminated with o-cresol. Thethioxylenols generally contain contaminating amounts of close-boilingcresols and xylenols. This process is particularly useful for purifyingtar acidcontaminated thiophenol, thiocre'sols, and thioxylenols.

The alkoxides of aluminum and magnesium are readily prepared by reactingthe metal, preferably in granular form and moisture-free and oxide-free,with a stoichiometric excess of the alcohol, preferably under reflux conditions. Several methods are known for preparing magnesium and aluminumalkoxides. The primary alcohols are the more reactive, there generallybeing a marked decrease in the reactivity of the hydroxyl hydrogen ofthe alcohol in proceeding from a primary to secondary to tertiaryalcohol. Thus with the tertiary alcohols the temperature must beincreased and the reaction time lengthened. While in some instances itmay be desirable to catalyze the reaction by amalgamating the magnesiumor aluminum, this may introduce subsequent problems of removal of themercury. Other catalytic techniques are'therefore preferred. Aluminumisopropoxide, which is commercially available, may be readily preparedby treating aluminum wire or'foil with anhydrous isopropyl alcohol,using carbon tetrachloride as catalyst. The contents are protected frommoisture by a mercury trap or a calcium chloride drying tube. Themixture is then refluxed until all the aluminum has dissolved. Theresulting solution is then distilled under reduced pressure.

Excess isopropyl alcohol distills over first, after which the condenseris removed, since air cooling is suflicien't for the condensation ofaluminum isopropoxide. Other lower alcohol alkoxides may be similarlyprepared charged to a batch still, and at least one third of a gramatom, based on aluminum or magnesium content, is added as an aluminumalkoxide or magnesium alkoxide per gram mole of tar acid present in thecharge. An excess of available aluminum or magnesium above thestoichiometric amount is ordinarily not required for substantiallycomplete removal of the tar acids if moisture is not present. Thepresence of excess aluminum alkoxide beyond that necessary for reactionwith the tar acids and moisture is preferably avoided as the aluminumalkoxides are less reactive than the magnesium alkoxides with respect tothiophenols, and may undergo a slow interchange with the thiophenolsthereby continuing to generate the related alcohol even while thethiophenol is being distilled. This may contaminate the thiophenoldistillate and require removal of the alcohol by further fractionationof the tar acid-free thiophenol. The charge is heated at atmosphericpressure, preferably to a temperature between 100 and 220 C., dependingon the specific thiophenolic and phenolic contaminants that are present.The lower molecular weight phenols and thiophenols readily react at thelower end of the temperature range.

- A preferred reaction temperature range for a thiophenol-phenol mixturein which vigorous reaction occurs is between 135 and 165 C. For mixedthiocresols, a range'between 140 and 185 C. is preferred. The mixedthioxylenols have a preferred reaction range between 160 and 200 C.Where substantial amounts of tar acids are present, in excess of 20percent by weight, vigorous reaction occurs at the lower end of therange and even atlower temperatures. Because of the reaction between themagnesium and aluminum alkoxides with the tar acid is an exothermic one,these alkoxides may be added to a phenol-thiophenol mixture at roomtemperature, or lower, the heat of reaction gradually raising thetemperature of the system. Upon addition to the mixture, the solidalkoxide almost immediately begins to dissolve therein, the evolution ofheat promoting its solution. This re action may therefore occur over atemperature range from to 200 C. because of its exothermic nature. Thereaction is ordinarily completed within several seconds to severalminutes, depending upon the initial temperature employed and therelative amount of tar acid present. In' the course of the reactionthere is no evolution of hydrogen. Rather an interchange of aluminum ormagnesium occurs, so that upon use of an effective, i.e., moisture-free,stoichiometric amount of aluminum alkoxide or magnesium alkoxide, allthe tar acids present are combined with the aluminum or magnesium.Higher reaction temperatures above room temperature are ordinarilyemployed only because of the subsequent temperature requirements fordistillation of the particular thiophenol being purified. Distillationis preferably carried out under vacuum at pressures of 50-100 mm. Hg.

7 This process is particularly applicable to purifying thiophenolscontaining tar acids as recovered from the paraflinic naphtha fractionobtained from the doublesolvent extraction method of treatingcaustic-extracted petroleum distillates, as set forth in US. Patent2,767,- 220. However, the process is equally applicable to refiningthiophenols contaminated with other substances in addition to tar acidsin that these other contaminants are removed by fractionation eitherbefore or after treatment of the thiophenol with the alkoxide.

The process is also considered applicable to the removal of moisture,particularly in trace amounts, as well as phenols, from thiophenols. Ithas been found that, in general, aluminum and magnesium alkoxides becomeselectively less reactive in going from water to phenols to thiophenols.Inasmuch as an alcohol is formed when moisture and tar acids are removedby reaction with the alkoxide, the latter should be selected so that adesiredalcohol is formed. Either the quantity and particular alcohol,formed should be noncontaminating, or

"ide is formed. This may then be partially separated from thethiophenols by filtration or centrifugation. However, it is ordinarilypreferred to separate the thiophenols from the realtively nonvolatilealuminum phenoxides by distilling oflf the thiophenols from the mixturewithout any prior cooling of the mixture.

The following examples illustrate this invention but are not intended aslimitations thereof.

EXAMPLE 1 Aluminum isopropoxide reaction A synthetic mixture of 9.8grams (0.104 mole) of phenol and 88.8 grams of thiophenol was treatedwith 15.2 grams (0.074 mole) of aluminum isopropoxide. The aluminumisopropoxide used as catalyst was a commercially obtained sample. Thetemperature was raised to C. over a period of 0.5 hour. The pressure wasthen reduced to 50 mm. Hg, and a 94 percent recovery of phenol-freethiophenol was achieved. The initial dis- 'tillate consisted of amixture of thiophenol and all of the isopropyl alcohol formed.

Excess amounts of the aluminum isopropoxide beyondthat necessary forreaction with the tar acids and moisture undergo a slight interchangewith the thiophenols thereby regenerating isopropyl alcohol. Thisalcohol may distill over with the thiophenols in detectable though verysmall amounts. Use of a smaller excess of reagent where the'. feedcomposition is accurately known would minimize this. Excess alcoholpresent in the initial distillate may be removed by refractionating thetar acid-free thiophenols.

EXAMPLE 2 Magnesium methoxide reaction An amount of 15.5 grams (0.63gram atom) of 70-80 mesh granular magnesium and a stoichiometricquantity of dry methanol were refluxed for 4.5 hours. A vigorousreaction occurred at that time to yield a gray, freeflowing powderconsisted of magnesium methoxide and unreacted magnesium. Hydrogen gaswas evolved in the process. The initiation period of the reaction isconsiderably diminished if a small amount of the preformed alkoxideproduct is added to the reaction mixture. Although magnesium metal perse has not been found to react with a mixture of thiophenols and taracids, the unreacted magnesium metal mixed with its methoxide salt willbe activated and react.

The reagent as prepared above was added to a synthetic mixture of 31.0grams of phenol (0.33 mole) and 301 grams of thiophenol. The pressurewas reduced to 50 mm. and the distillation was carried out. Methanol wasremoved followed by 96 percent of phenol-free thiophenol.

It has been found that the magnesium alkoxides are much more reactivethan the aluminum alkoxides with respect to the thiophenol present. Thusunlike aluminumisopropoxide, which reacts very rapidly with tar acidsand much more slowly with thiophenols, magnesium methoxide interchangesrapidly and irreversibly with tar acids and almost as rapidly withthiophenols. no magnesium methoxide remains unreacted in the still poteven where a large excess is employed, sinceany excess is converted tothe thiophenoxide salt.

formed is readily removed in the distillation fore-cut;'

Hence Where. an excess of magnesium methoxide is used, the methanolsucceeding thiophenol fractions are therefore free of methanol. Thethiophenol recovered is both tar acidfree and alcohol-free.

In general, While higher alcohol derivatives of magnesium and aluminummay be employed, it is preferred to employ the alkoxides of loweraliphatic alcohols, those having from 1 to 4 carbon atoms. Use of theselower alcohols offers the advantage that the boiling point of thealcohol is generally Well below that of the thiophenol in which it maybe present. Thereby a separation of the alcohol in a fore-cutdistillation is readily attained. The use of an alcohol having a higherboiling point than the thiophenol being purified would be undesirable.Furthermore, because the metal, namely the aluminum and magnesium, isessentially the elfective reactant, the use of lower-alcohol-derivedalkoxides allows a greater proportion of metal present for a givenmolecular weight of alkoxide. For example, one pound of magnesiummethoxide is equivalent to 3.15 pounds of aluminum isopropoxide becauseof the lower chemical equivalent weight of the magnesium methoxide. Thusaluminum isopropoxide priced at 30' cents per pound would be conomicallyequivalent to magnesium methoxide priced at one dollar per pound.

The interchange reaction that occurs between the magnesium and aluminumalkoxides with the thiophenol and phenol is considered a highly complexone. While I do not desire that the scope of this invention berestricted by any explanation profiered, it is considered apparent thatthe formation of the aluminum and magnesium phenoxides proceeds at amuch more rapid rate than that of the corresponding aluminum andmagnesium thiophenoxides. The magnesium thiophenoxides are, however,much more rapidly formed than aluminum thiophenoxides undercorresponding conditions. Also, tar acids appear much more reactive thanalcohols with respect to aluminum and magnesium. Thus aluminum andmagnesium phenoxides are much more readily formed than the correspondingalkoxides, and hence displacement of the aluminum and magnesium fromthese alcoholates is essentially irreversible. Under the conditionsused, the reaction is therefore highly selective to the removal ofphenol from a phenol-thiophenol mixture. This selectivity phenomenonoccurs despite the fact that thiophenol is a considerably stronger acidthan phenol, and hence the thiophenol might ordinarily be expected to bemore reactive than the phenol. However, as shown herein, not only is thereaction selective, but apparently the aluminum and magnesium will reactwith the phenol to almost the complete exclusion of the thiophenol aslong as any phenol is present.

While this invention has been described with respect to specificpreferred embodiments, I do not desire to be limited by the illustrativeexamples given or by the speculative mechanisms postulated for thisreaction, but the scope of this invention should be determined inaccordance with the objects and claims thereof.

I claim:

1. The process for purifying a tar acid-contaminated thiophenol whichcomprises reacting a mixture of a thiophenol and a tar acid with analkoxide selected from the group consisting of aluminum and magnesiumalkoxides to react selectively with the tar acid to form a correspondingmetal salt thereof and an alcohol of said alkoxide, and separating thethiophenol from said salt.

2. The process according to claim 1 wherein said metal alkoxide is asalt of a monohydroxy parafiinic alcohol having from 1 to 4 carbonatoms.

3. The process for recovering a thiophenol in substantially pure formfrom a mixture containing a thiophenol and a tar acid which comprisesadding to said mixture an amount of a monohydroxy parafiinic alkoxidehaving from 1 to 4 carbon atoms and selected from the group consistingof aluminum and magnesium alkoxides sulficient to provide at least gramatom of metal per mole of tar acid present to react selectively with thetar acid to form a corresponding metal salt thereof and an alcohol ofsaid alkoxide and recovering the thiophenol from the mixture insubstantially pure form free from tar acid and alcohol.

4. The process according to claim 3 wherein said alkoxide is aluminumisopropoxide.

5. The process according to claim 3 wherein said alkoxide is magnesiummethoxide.

6. The process for recovering thiophenol per se in substantially pureform from a mixture containing thiophenol and phenol which comprisesadding under reactant conditions an amount of magnesium methoxidesufiicient to provide at least /3 gram atom of metal per mole of phenolpresent to react selectively with the phenol to form magnesium phenoxideand methyl alcohol, distilling said mixture to recover substantially allsaid methyl alcohol as a fore-cut, and continuing said distillation torecover thiophenol in substantially pure form free from phenol andmethyl alcohol.

No references cited.

1. THE PROCESS FOR PURIFYING A TAR ACID-CONTAMINATED THIOPHENOL WHICHCOMPRISES REACTING A MIXTURE OF A THIOPHENOL AND A TAR ACID WITH ANALKOXIDE SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND MAGNESIUMALKOXIDES TO REACT SELECTIVELY WITH THE TAR ACID TO FORM A CORRESPONDINGMETAL SALT THEREOF AND AN ALCOHOL OF SAID ALKOXIDE, AND SEPARATING THETHIOPHENOL FROM SAID SALT.